1. Field of the Invention
The present invention relates generally to defibrillation methods, and more particularly, to a system that is able to deliver a true voltage pulse to the heart from a charged capacitor.
2. Description of the Prior Art
Defibrillation, or causing the cessation of chaotic and uncoordinated contraction of the ventricular myocardium by application of an electrical direct current and voltage, in its most primitive form, goes back to the last century. [J. L. Prevost and F. Batelli, "Sur Quelques Effets des Descharges Electriques sur le Couer des Mammifers", Comptes Rendus Hebdomadaires des Seances de L'Acadmie des Sciences, Vol. 129, p. 1267, 1899.] Because of the large currents required for defibrillation, large-area electrodes are employed. [A. C. Guyton and J. Satterfield, "Factors Concerned in Defibrillation of the Heart, Particularly through the Unopened Chest", Am J of Physiology, Vol 167, p. 81, 1951.]
For reasons of simplicity and compactness, capacitor-discharge systems are almost universally used in defibrillation. The discharge of a capacitor C through a resistance R results in a pulse that is a declining exponential function, with a characteristic time given by the product RC. But it has also been recognized for some time that the long-duration, low-amplitude "tail" of the capacitor-discharge pulse is detrimental. [J. C. Schuder, et al., "Transthoracic Ventricular Defibrillation with Triangular and Trapezoidal Waveforms", Circ. Res., Vol. 19, p. 689, October 1966; W. A. Tacker, et al., "Optimum Current Duration for Capacitor-discharge Defibrillation of Canine Ventricles", J. Applied Physiology, Vol 27, p. 480, October, 1969.] The exact reason for this detrimental effect is not known, although plausible speculations exist, with one possibility being that field heterogeneities cause arrythmias in significantly large regions of the heart. [P. S Chen, et al., "The Potential Gradient Field Created by Epicardial Defibrillation Electrodes in Dogs", Circulation, Vol. 74, p. 626, September 1986.] A convenient way to eliminate the low-amplitude "tail" of a capacitor discharge is by switching, which is to say, simply opening the capacitor-load circuit after a predetermined time, or else when voltage has fallen to a particular value. For this reason, the time-truncated capacitor discharge has been extensively used after its effectiveness was first demonstrated. [J. C. Schuder, et al., "Transthoracic Ventricular Defibrillation in the Dog with Truncated and Untruncated Exponential Stimuli", IEEE Trans Biom. Eng., Vol. BME-18, p. 410, November 1971.]
The defibrillation effectiveness of time-truncated capacitor discharges can be convincingly shown by comparing an untruncated waveform and a truncated waveform of equal effectiveness. The full-discharge waveform of FIG. 1A was generated by charging a 140-F capacitor to 655 V, for an energy delivery of 30 J. But the truncated waveform shown in FIG. 1B, known in the art as a monophasic waveform, was equally effective for defibrillation in spite of having only half the energy, and a lower initial voltage. Similar results have been obtained for the case of dogs using a catheter electrode and a subcutaneous patch [M. Mirowski, et al., , "Standby Automatic Defibrillator", Arch Int. Med., Vol. 126, p. 158, July 1970.], as well as with a dual-electrode intraventricular catheter. [J. C. Schuder, et al., "Ventricular Defibrillation in the Dog with a Bielectrode Intravascular Catheter", Arch Int. Med., Vol. 132, p. 286, August 1973.] The latter electrode arrangement was also used to demonstrate the point for the case of man. [M. Mirowski, et al., "Feasibility and Effectiveness of Low-energy Catheter Defibrillation in Man", Circulation, Vol 47, p 79, January 1973.]Such demonstrations that compact capacitor-storage systems could be used with effectiveness paved the way for implantable defibrillator systems.
In spite of the dramatic results obtained with time-truncated capacitor-discharge defibrillator systems, the waveform specifications have not been systematically optimized. Present art arranges to have external current to the heart go to zero at the time of truncation, a condition that significant evidence shows to be less than optimum. The zero-current condition is a result of the switch configuration conventionally used for generating a monophasic pulse, which is shown in FIG. 2. Each switch symbol represents a solid-state device, usually a thyristor. This device, earlier known as a "silicon controlled rectifier", exhibits turn-on gain, but not turn-off gain. That is, a relatively low-power control pulse applied through an extra terminal is able to switch the device from its blocking, or OFF, condition to its conducting, or ON, condition. Turning the device off again requires reducing the current through it, or voltage across it, to a low value. The diagram of FIG. 2 is highly schematic, omitting such known details as small current-limiting resistors.
In operation of this system, the capacitor C is maintained in the charged condition by the high-voltage circuitry at the left. Then, the series switch 36 of FIG. 2 is turned ON to initiate the monophasic pulse, causing the capacitor C to commence its discharge through the heart. At the truncation point, the shunt switch 42 is closed, causing the capacitor C to be discharged rapidly. Consequently, the SCR will be back-biased and will turn off. The current from the capacitor to the heart will also decline rapidly, along with the decline in voltage, approaching zero, and reaching zero after the series switch 36 opens. The result is the monophasic waveform illustrated in FIG. 3A.
The prior art has sometimes used devices other than thyristors for the high-current switching. There are, for example, variations on the thyristor that exhibit a measure of turn-off gain, as well as turn-on gain, but these require more elaborate control circuitry than does the standard thyristor. Also there are available more complex devices that are in effect compound transistors, combining MOSFET and BJT principles, thus requiring less straightforward design. Pure field-effect devices, such as power MOSFETs, can handle currents of the order of those delivered to the heart in a defibrillation pulse. But note that the capacitor-discharging function of the shunt switch 42 involves much large instantaneous currents than that. Devices based upon carrier injection, such as the thyristor, are better adapted to this high-current requirement than are MOSFETs, which are essentially resistive devices.
It is known in the prior art that the heart-cell stimulation resulting from high-current delivery can be augmented by the sudden removal of current, a phenomenon known as "break stimulation." It is also known that break stimulation is further enhanced by actually reversing current, rather than simply reducing it to zero. A demonstration of this was provided when workers studying the effectiveness of multiple defibrillation pulses observed that a waveform consisting of a pair of contiguous pulses of opposite polarity was more effective than a monophasic pulse. Such a waveform is known as biphasic, and is well-established in the prior art. [Bach, U.S. Pat. No. 4,850,537, and Baker, U.S. Pat. No. 4,821,723.] The biphasic waveform is illustrated in FIG. 3B. Unfortunately, the prior-art biphasic system requires elaborated switching and control arrangements. Hence there is a need for a system that is capable of delivering current reversal with circuitry as simple as that of the monophasic system.